1. Field
The following description relates to data transmission between a wireless sound transmitter and a wireless sound speaker that outputs, and more particularly, to a method for compensation of a play time delay between the wireless sound speakers that occurs when the multichannel sound data is distributed and transmitted from the wireless sound transmitter to the multiple wireless sound speakers.
2. Background
Home theaters are vastly popular in the home environment. The term ‘home theater’ was used before the digital versatile disks (DVD) were commonly used, but it has been generalized after DVD systems became more widely used. A DVD system provides an excellent picture quality and a realistic sound for a user.
DVD has an advantage in that it can provide an exceptionally realistic sound compared to that of a laser disk (LD), a video home system (VHS), or a video compact disk (VCD). For example, in a scene from the movie ‘Saving Private Ryan,’ when a bullet is flying forward from behind, the sound played by the speaker system in the theater actually moves forward from behind simulating the direction of the bullet. In the movie ‘Matrix,’ when a helicopter is hovering above the head of the actor, the sound played by the speaker system in the theater makes viewers feel as if the helicopter is really hovering above their heads. Such three dimensional sound effect may be realized by multichannel digital sound technology. The most commonly used multichannel digital sound nowadays is digital multi-channel sound, for example, 5.1-channel digital sound or 6-channel digital sound.
5.1-channel is comprised of five channels including left and the right front channels located in front of a user, left and the right rear channels located in rear of the user, a center channel, and a 0.1-channel subwoofer that enhances a low frequency region. The speakers for the front channels that are located in front of the viewers are called main speakers. The role of the main speakers is to cover the background music and sound effect connecting the left and the right directions in front of the listeners.—
Typically, larger speakers are used as the main speakers as compared to the other speakers because they are advantageous when playing the stereo channels, but the main speakers are not necessarily larger than the other speakers. A speaker for the center channel, called a center speaker, covers dialogue and the human voice, i.e. vocal sounds, and it delivers a certain level of music and/or sound effects to the listeners. Occasionally, the center speakers are installed above the television so that the sounds are heard as if the actor on the screen are talking to the viewers. The speakers for the rear channel are called rear speakers or surround speakers and are usually installed at the both sides of the rear area of the listeners. The rear speakers mainly cover the surround effects. In past times mono surround channels were used, but nowadays stereo surround channels are used for 5.1-channel systems.
Meanwhile, the subwoofer covers a low frequency region and has very little restriction on its position because directivity at low frequencies is lower than directivity at medium or high frequencies. Most subwoofers are able to play 20 to 30 Hz but high performance subwoofers are able to play sounds lower than 20 Hz. Such low frequency sound provides not only the sounds but also a feeling of vibration thus making a more realistic sound play.
In some embodiments, the subwoofers have amplifiers therein. Recently, active type subwoofers having built-in amplifiers are becoming more popular for obtaining powerful sounds and automatic volume control.
The above mentioned speakers, except the subwoofer, are generally not physically different than each other but have different locations depending on their roles in the system. In some embodiments, the center speaker has slightly different characteristics for proper processing of the dialogues in a movie.
There are various types of multi-channel digital sound play system, for example, 5.1-channel system, such as 4-channel system, 4.1-channel system, and the like. Recently, 6.1-channel surround home theater systems are being introduced. When installing a home theater having multi-channel digital sound play system, each speaker must be connected to the corresponding terminals for signal output.
A typical wireless distribution and transmission method for a multichannel sound transmission is a time division multiple access (TDMA) method. The TDMA based multi-channel wireless speaker system illustrated in FIG. 1 adopts a method wherein each sound data is transmitted during the time slot assigned for each speaker so as to avoid inter-channel interferences.
In TDMA, M number of multichannel sound data extracted from the sound source are packetized in an appropriate time frame for outputting via wireless sound speakers (hereinafter referred to as “WSS”), then transmitted via wireless channels to corresponding speakers during the total transmission period through the modulator further adding play time information including speaker identification, current time, play time, and the like.
Each WSS outputs sounds through the speaker after extracting sound data by demodulating the wireless signal received. In TDMA, because multichannel sound data is continuously transmitted in temporal perspective, synchronization based on current time and play time must be performed between the speakers after all the channel signals are received in order to play sounds at the same time in all WSSs. For this reason, a buffer that is able to store sound data of more than two packets is required for continuous play, and a structure for synchronization between the WSSs should be added.
For synchronization between the WSSs, current time information must be periodically transmitted from the wireless sound distributer and transmitter (hereinafter referred to as “WSDT”), and all the packets must be transmitted with play time information. In each WSS, a reference time is set by receiving current time information and based on a reference time. Sound play should be performed in accordance with play time information of each packet. Such a structure requires an additional storage space, a timer, and a play time controller because a received packet must be stored in a buffer until the play time point. This causes an increase in hardware complexity, and it has further disadvantages in that transmission efficiency is reduced because additional information for synchronization between the speakers must be added to all the packets.